The Suzuki-Miyaura reaction is a palladium- or nickel-catalyzed cross coupling between a boronic acid or a boronic ester, and an organohalide or an organo-pseudohalide. (Miyaura, A. Chem. Rev., 1995) This cross coupling transformation is a powerful method for CC bond formation in complex molecule synthesis. The reaction is tolerant of functional groups, and has become increasingly general and widespread in its use for coupling of organic compounds. (Barder, 2005; Billingsley, 2007; Littke, 2000; Nicolaou, 2005)
Organoboronic acids are notoriously sensitive to many common reagents. (Hall, 2005; Tyrell, 2003) It is therefore typical to introduce the boronic acid functional group during the last step of a building block synthesis. However, many of the methods for doing so (hydroboration, trapping organometallic reagents with trimethylborate, etc.) are intolerant to a variety of common functional groups, such as alcohols, aldehydes, ketones, alkynes and olefins. This makes the synthesis of structurally complex organoboronic acid building blocks quite challenging.
One area of research on the Suzuki-Miyaura reaction is the development of protecting groups for the boronic acid functional group. A compound that includes a protected boronic acid group and another functional group can undergo chemical transformations of the other functional group without chemically transforming the boron. Removal of the protecting group (deprotection) then provides the free boronic acid group, which can undergo a Suzuki-Miyaura reaction to cross-couple the compound with an organohalide or an organo-pseudohalide.
In one example of a boronic acid protecting group, each of the two B—OH groups is converted into a boronic ester group (>B—O—R) or a boronic amide group (>B—NH—R), where R is an organic group. The organic group can be removed by hydrolysis to provide the free boronic acid group. (Deng, 2002; Hohn, 2004; Holmes, 2006; Noguchi, 2007) The heteroatom-boron bonds in these protected compounds tend to be very strong, however. The relatively harsh conditions required for cleaving these ligands typically are incompatible with complex molecule synthesis. In another example of a boronic acid protecting group, three organoboronic acid molecules can be condensed to form a cyclic boroxine. (Kerins, 2002) These protected organoboronic acids, however, tend to be unstable to long term storage.
The reactivity of a boronic acid group also may be decreased by conversion of the boronic acid group into a tetracoordinate anion, such as [R—BF3]−, where R represents an organic group. These anions may be present as salts with a counterion, such as K+ or Na+. (Molander, 2007) Another class of tetracoordinate boron anions, [R—B(OH)3]−, has been reported in the context of purifying organoboronic acids for use in the Suzuki-Miyaura reaction. (Cammidge, 2006) In each of these systems, the boron itself is not protected from the Suzuki-Miyaura reaction, but can be used directly in the coupling transformation.
Recently, it was discovered that N-methyliminodiacetic acid (MIDA) boronates can be used to protect boronic acid functional groups from a variety of chemical reactions. (Gillis, 2007; Lee, 2008) These MIDA boronates are stable to air and to purification by chromatography, and do not cross-couple under anhydrous conditions. However, the MIDA boronates can be hydrolyzed rapidly with aqueous NaOH (<10 minutes at 23° C.) to release the corresponding unprotected organoboronic acid. Thus, MIDA boronates can be used as convenient surrogates for stable organoboronic acids under aqueous NaOH-promoted Suzuki-Miyaura coupling conditions. This approach has been shown to be effective for both aryl- and alkenyl-MIDA boronates. The only change to the standard coupling protocol is the inclusion of extra base to hydrolyze the MIDA boronate. The cleaved MIDA2− ion appears to have no deleterious effect on these reactions, even though it is known to be a ligand for Pd(II).
The published synthetic methods for protection of organoboronic acids as MIDA boronates have been less successful when used to protect unstable organoboronic acids. Unstable organoboronic acids are susceptible to degradation, which can preclude their benchtop storage and/or their efficient cross-coupling. When MIDA boronates of unstable boronic acids are simultaneously deprotected and reacted with an organohalide or organopsuedohalide under NaOH-promoted Suzuki-Miyaura coupling conditions, the yield of cross-coupled product can be undesirably low. Examples of unstable organoboronic acids include 2-heterocyclic boronic acids (Billingsley, 2007; Tyrrell, 2003), vinyl boronic acids (Matteson, 1960), and cyclopropyl boronic acids (Wallace, 2002). Although unstable organoboronic acids can be successfully prepared and stored in the form of their corresponding MIDA boronates, the deprotected organoboronic acids may be susceptible to degradation during their subsequent use.
It would be desirable to perform high yield cross-coupling reactions using MIDA boronates of unstable organoboronic acids. Preferably the cross-coupling would be as selective and as tolerant of functional groups as a conventional cross-coupling reaction. It would also be desirable for such a reaction to be efficient and simple.